The 400ha site at Gillman, in Adelaide's north, has been offered to Adelaide Capital Partners for $100 million, with the company given six months to come up with a plan before committing to the purchase.

The government has argued that the proposal will deliver a piece of economic infrastructure where the developer accepts the risk, pays for the land and creates thousands of jobs.

But independent upper house MP John Darley says the sale should have been put out to tender and wants a parliamentary inquiry to look into the deal.

He has the support of the Liberal opposition with housing and urban development spokeswoman Vickie Chapman saying the government's actions require further scrutiny.

"South Australians need to know whether this deal was in their best interests," Ms Chapman said.

Mr Weatherill cautioned those conducting any parliamentary investigation to proceed carefully or risk scuttling the proposal.

"They should be careful to conduct themselves in a fashion that doesn't scare off the international investors that are necessary to make that deal a reality," he said.

"There has been no concluded contract in relation to Gillman. It's only a question of investigation of an option to purchase."